This application claims the priority of German patent document 196 45 385.2, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a spark plug which generates a spark that jumps between two electrodes.
Spark plugs have two electrodes, between which a spark jumps upon application of an ignition voltage that exceeds the electrical spark-over voltage, to ignite a flammable gas mixture surrounding the electrodes. Usually a ground electrode of the spark plug is formed on an electrically conducting housing that has an insulating body with a center electrode therein. In known spark plugs, the ground electrode is separated from the center electrode by a short distance (approximately 0.4 to 0.8 mm). When the spark voltage is applied, a spark that causes the mixture to ignite jumps between the center electrode and the ground electrode. Because of the narrow spark gap between the electrodes, known spark plugs can reliably ignite only flammable monatomic and homogenous mixtures. That is, if an inhomogeneous gas mixture with local differences in concentration is present, no ignition can occur if the spark jumps in a portion of the mixture where there is a surplus of air and a shortage of the fuel component.
The disadvantages of the narrow spark gap between the electrodes of conventional spark plugs are manifested in the operation of direct-injected gasoline engines with charge stratification, such as those disclosed for example in German patent document DE-OS 43 24 642 A1. In this case, a mixture cone is formed from a conical fuel stream from an injector, with concentration gradients that decrease outwardly, and the electrodes of the spark plugs project into the mixture cone. An ignitable mixture is present at the electrodes of the spark plug in the outer area of the mixture cone, but this mixture is leaner than the mixture composition near the injector, so that fuel consumption advantages and lower pollutant emissions from the engine are achieved during subsequent stratified combustion. Variations in the injector nozzle as a result of manufacturing methods, or even deposits which develop with increasing operating time of the engine, cause a skeining of the fuel stream, so that fluctuations in concentration occur especially in the jacket area of the mixture cone. If the fuel concentration of the mixture present at the electrodes at the moment of injection is below the limit of ignitability, the spark that jumps between the electrodes cannot initiate the combustion process, causing a misfire. Such misfires should be avoided at all costs because of the rough engine operation that results, as well as the high pollutant emissions from the engine.
An object of the present invention therefore, is to provide a spark plug of the type described above which achieves improved combustion of flammable gas mixtures with locally different mixture concentrations.
Another object of the invention to locate the spark plug in such fashion in a gasoline engine, especially an engine with charge stratification, that its normal operation is improved.
These and other objects are achieved by the spark plug according to the invention, in which the center electrode projects beyond the ground electrode in the axial direction, so that a long spark gap can be formed that matches the distance between the electrodes. With this configuration, flammable gas mixtures with locally different concentrations can be ignited reliably. The spark passes through the mixture on a precisely defined path between the electrodes, so that it reliably encounters at least a partial volume of the mixture which is within the ignition limits. The path of the spark from the center electrode to the ground electrode then passes close to the sections of the spark plug that are heated to the greatest extent, so that the ignitability of the portion of the mixture that is located between the electrodes is improved because of the higher temperature.
A longer yet controlled spark path can be achieved because the insulating body extends in the axial direction of the center electrode for at least the same distance as the ground electrode on the housing. Although the adjacent insulating body itself is not a conductor, because of its mass and the high ignition voltage the spark will first jump to the insulating body and will be conducted from there to the ground electrode. Thus two partial spark paths are formed which together correspond to the total spark gap between the electrodes. Advantageously the insulating body is surrounded by the housing of the spark plug with a radial gap, forming a chamber that is open to the electrodes, so that greater heating of the insulating body and the center electrode enclosed therein is achieved. In addition, the chamber has a heat-insulating effect and prevents the insulating body from being cooled as a result of the conduction of heat through the housing.
The spark plug is preferably used for igniting mixtures in gasoline engines, with at least one spark plug having its electrodes projecting into the combustion chamber of each cylinder and thus into the mixture contained in said cylinder. The housing of the spark plug is electrically connected with the cylinder head, thus ensuring the conduction of the igniting spark in addition to holding the spark plug in place. If the engine operates with internal carburetion and charge stratification, the stratified inhomogeneous fuel/air mixture in the combustion chamber can be ignited reliably. Advantageously, at least the center electrode penetrates the outer jacket of a mixture-guiding stratification cone each time fuel is injected into the combustion chamber. The stratification cone is formed by the conical stream during injection by an injector located in the cylinder head, and, due to the stratification, has .lambda. values that increase toward the outside starting at the center of the cone with the highest fuel concentration.
In the vicinity of the outer jacket, the mixture composition is within the ignition limits, with the nature of stratified charge operation being such that the skeining of the conical stream that is injected produces a lean mixture locally in the outer jacket of the mixture cone, such composition being below the ignition limit. These nonflammable partial volumes with high .lambda. values are small however, and because of the long spark travel distance of the spark plug, starting from a center electrode that projects into the mixture cone, the ignition spark that jumps between the electrodes during each ignition process passes through a partial volume of the mixture with .lambda. values within the ignition limits. If the center electrode corresponding to the location of the spark plug in the cylinder head is pointed at the axis of symmetry of the mixture cone, the spark gap extends between the electrodes in a mixture area with the largest possible concentration gradient, and stratified combustion can be triggered extremely reliably by the igniting sparks.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.